The invention relates to the luminal endoprostheses formed principally of a framework, without textile covering, generally called “stents”, and more particularly to the stents for blood vessels.
Over the years, the implantation of luminal endoprostheses has become an approved technique for treatment of aneurysms, atherosclerosis, etc.
However, one crucial problem has still not been solved: namely that of perfectly matching the mechanical characteristics of these endoprostheses and of the organs in which they are implanted.
Even if very particular care has been taken to meet these criteria at the time of implantation, a disparity invariably develops in the long term. This is because the human body is subject to changes due to aging, while the endoprosthesis has a problem of stability over the course of time: tearing of the filaments, deterioration of the structure, possible increase in diameter (by loosening of the structure).
The mechanical characteristics of a stent are determined essentially by the structure of its framework. Although different types of these exist, such as the frameworks made up of flat braids described in WO 99/55256, the most suitable framework at present is the cylindrical braided framework, such as is described in particular by Didcott in GB-1205743, or in U.S. Pat. No. 5,061,275.
This type of framework compresses easily for insertion, resists well to crushing and retains a relative flexibility compatible with that of the blood vessels; the structure adapts to the sinuous course of the rigid arteries to be treated.
To date, investigations into finding the optimum framework have focused on the choice of material, the braiding pitch, etc.
These investigations inevitably come up against a number of practical problems.
By adopting a very small braiding pitch (the angle between the axis and the spirals being close to 90°) or by choosing thick wires, the radial force (resistance to crushing) is increased, but flexibility is lost. This problem is even more critical for stents and endoprostheses made up of several modules cut by laser.
Conversely, a large pitch, where the angle formed between the axis and the spirals is close to 30° for example, and the use of thin wires give the framework good flexibility but a low resistance to crushing. In addition, such a pitch signifies a considerable rate of shortening at the time of implantation of the endoprosthesis, which entails a lack of precision upon deployment.
Attempts have been made, particularly in EP-0 804 909, to combine metal wires with textile fibres. This technique is aimed exclusively at the treatment of aneurysms and cannot be applied, for example, to treatment of stenosis of the carotid or even femoral artery. However, the results obtained are not convincing: the metal filaments deform the structure and, along their helical course, they create dislocations and spacing of the textile fibres. The fibres are subjected to stresses under the effect of the pulsations caused by the blood flow and they are subject to rapid erosion by friction against the metal filaments (whose modulus of elasticity and diameter are greater).
Results based on recent clinical studies have shown that, in the case of an aneurysm of the abdominal aorta, 70% of the pressure wave is transmitted to the wall of the aneurysm via the endoprosthesis. (Reference: Communication at the 27th Global Vascular Endovascular Issues Techniques Horizons™ Nov. 16-19, 2000, page V5.1). These findings are not surprising because haemodynamics teach us that when the walls are thin, the work necessitated by the transport of the blood increases. It is also known that when the vessels are too large, the volume of blood increases beyond what is necessary. These factors promote aneurysms. This shows that more stable and more robust structures have to be developed. This is the object of the invention described below.